clocking for medical ultrasound systems

1SNAA311–September 2017Submit Documentation Feedback

Copyright © 2017, Texas Instruments Incorporated

Clocking for Medical Ultrasound Systems

Application ReportSNAA311–September 2017


Ravindra Munvar, Christian Schmoeller, Julian Hagedorn, Leni Skariah, Arvind Sridhar

ABSTRACTThis application note discusses the importance of clocking in Ultrasound and also illustrates how somekey TI devices achieve very low end to end jitter and phase noise. The application note also demonstrateshow various stages have very low additive jitter.

Contents1 Introduction ................................................................................................................... 22 Clock Tree .................................................................................................................... 43 Summary and Conclusion .................................................................................................. 8

List of Figures

1 Front End Unit and Back End Unit ........................................................................................ 42 Front End Alone.............................................................................................................. 43 Different Clocking Components............................................................................................ 54 Demonstration 1, LMK61E2 ................................................................................................ 55 Demonstration 2, LMK04803............................................................................................... 66 Demonstration 2, LMK04610............................................................................................... 67 Demonstration 3, LMK0030X .............................................................................................. 68 Comparison of LMK04803 to LMK0030X................................................................................. 79 Comparison of LMK0461X Series ......................................................................................... 710 LMK0461X Frequency Combinations ..................................................................................... 7

List of Tables

1 Clock Requirement for 128-Channel Cart-Based Ultrasound System ................................................ 32 REF IN ........................................................................................................................ 4

TrademarksAll trademarks are the property of their respective owners.

http://www.go-dsp.com/forms/techdoc/doc_feedback.htm?litnum=SNAA311

Introduction www.ti.com

2 SNAA311–September 2017Submit Documentation Feedback



1 IntroductionMedical Ultrasound systems use Sound waves to generate images of the body. They do this bytransmitting an Ultrasound wave into the body and then receiving the echo. This echo is processed togenerate an image.

This transmission and reception of the ultrasound signals has to be highly synchronized – to achieve thebest results. Precision Clocking is crucial to ensure that the information displayed has minimum artifacts.

Phase noise (frequency domain) or Jitter (time domain) is a measure of how much a clock signal’s phaseor switching edge deviates from its ideal position and is an important parameter that can determine theimpact of reference clocks on the ultrasound image. For example, ultrasound often uses the DopplerEffect to determine the direction and magnitude of blood flow. Higher phase noise can lead to artifactsoccurring in Doppler images – such as speckles – which represent errors in the calculation and display ofblood flow. In Doppler imaging, the strong reflection from close surfaces and weak signals from higherdepths – results in a high dynamic range and jitter on the reference clock can potentially create undesiredartifacts.

TI’s high performance AFEs need a jitter performance of less than 800 fs to 1 ps to perform optimallyespecially in the CW Doppler mode.

There are various subsystems in the Ultrasound systems which need clocking:1. Front End:

(a) Transmit Pulsers – Need several copies of very low jitter, high frequency clocks in the range of 160MHz+ to achieve good beam focusing. (An example of ultra-low jitter reference source from TI isthe LMK61E2 family of crystal oscillators. Examples of ultra-low additive jitter buffers from TI thatcan be used to distribute low noise reference clocks are CDCLVC1310, LMK0030X, and so forth)

(b) Receive AFEs – Require several low skew copies of highly synchronized clocks for all the AFEs toensure that the data is sampled simultaneously. Frequencies are in the range of 40 to 80 MHz. (Anexample of a low skew distribution buffer family from TI is LMK0030X.)

(c) CW Doppler – Requires two very-low phase noise clocks which are crucial to determine thefrequency shift. Range from 2 to 128 MHz. (An example of a high performance dual-loop jitterattenuator/clock generator device family from TI suitable for generating CW Doppler clocks isLMK048xx or LMK0461x.)

(d) Beam-forming FPGAs – For transmit and receive beam-forming the clocks need to be highlysynchronized (LMK0030x fanout buffers). Transmit beam-forming focuses the ultrasound wave-front. Precise timing is required and any variation in the clock edges (jitter) can lead to loss offocus. The losses of precise synchronization between transmit and receive beamforming can alsoresult in loss of image resolution.

(e) FPGAs can also be used to generate the clocks needed for transmit and receive timing, but thejitter performance of these clocks is typically not nearly as good as specialized clocking ICs. Thejitter then needs to be cleaned by external jitter cleaners. (Examples of high-performance, dual-loopjitter attenuators from TI include LMK04610 and LMK0480x.)

2. Power Supply:(a) All the power supplies within the Ultrasound system are often synchronized to one master clock to

avoid the effects of switching noise appearing as artifacts in the image (through filtering). Thesepower supplies typically operate in the range of 100 kHz to 1 MHz. Higher switching frequenciesoften result in smaller magnetic components but also operate closer to the dynamic range of theultrasound signals themselves. Synchronizing all switching power supplies to the same (lower)frequency and then filtering at that frequency can avoid any coupling of this noise into theultrasound signal chain. In addition, spread spectrum clocking can help to reduce EMI effects.(Examples of flexible clock generators with spread-spectrum generation capability used to minimizeEMI in the system includes CDCE913 and CDCE6214)

(b) Once the signals are digitized and provided to the digital post-processing sub-systems, the jitter isthen not as critical. However it is desirable to have clean clocks throughout the entire system – toensure maximum performance and lower overall noise in the system.

http://www.ti.com


www.ti.com Introduction




3. Backend:(a) FPGAs – for image processing require frequencies which include the sampling frequency and

others needed for communication and internal logic blocks. (CDCM6208, LMK033x8, CDCE6214,LMK60xx)

(b) DSPs and Processors – for image processing – require common frequencies like 100 MHz.(CDCM6208, LMK03328, CDCE6214)

(c) External communication – PCI, PCI Express, USB, Ethernet, and so forth, that typically require24-, 48-, and 100-MHz clocks (CDCM6208, CDCE6214)

System designers prefer using clocks with the features as listed as follows:1. Low power dissipation (LMK04610, CDCE6214)2. Programmability and scalability – to support additional features or variations at a later date – without

change in hardware. (LMK61E2 reference oscillator, any LMK or CDC clock generator)3. High integration (less number of parts and smaller board real estate) (LMK04610, LMK04803,

LMK03328, CDCE913, and so on.)4. Fully-differential clocking wherever possible in the signal chain (common mode noise rejection).

Precision clocking can be achieved by having ultra-low noise clock sources, generators/dividers,distributors and buffers. The clocks generators are accurate and typically have lower phase noise wheninteger mode PLL and output dividers are used versus fractional mode PLL and/or fractional outputdividers – that could introduce additional phase noise and potentially some small frequency synthesiserror.

The impact can be minimized by ensuring that the front-end signal chain uses exclusively integer dividersto generate the clocks needed for the front end.

For the fanout of clock signals it is important to select buffers with very low additive jitter, otherwise thenoise floor of a high quality source clock (that is, LMK61E2) would degrade significantly each time it isbuffered. Suitable Buffers for this kind of application are the LMK0030x family (differential) and theCDCLVC1310 (LVCMOS), that have very low additive jitter.

TI’s clock generator, dividers and low jitter buffers are some key components which help achieve thisrequired performance.

Table 1 and the Figure 1 show a typical clock tree for an Ultrasound system. This clock tree demonstratesthe use of various reference clocks for different building blocks – Transmit, Receive, FPGAs, Processors,power supply, and so on.

Table 1. Clock Requirement for 128-Channel Cart-Based Ultrasound System

NO OF CLKS FREQUENCY FORMAT TARGET PERFORMANCE128 100 MHz LVCMOS DAC + AMP Clock Good Phase Noise2 40 MHz LVCMOS CW Mixer Good Phase Noise2 120 MHz LVDS FPGA (AWG)2 200 MHz LVDS FPAG (Transmit)1 200 MHz LVDS FPGA (Receive)16 100 kHz to 500 kHz LVCMOS Power Supply8 128 MHz LVDS AFE Good Phase Noise8 80 MHz LVDS AFE Good Phase Noise8 8 MHz LVDS AFE Good Phase Noise1 122.88 MHz LVDS DSP3 100 MHz LVDS DSP1 24.576 MHz LVCMOS Audio Codec

http://www.ti.com


CDCLVD1216 CDCLVD1216

LMK61E2

40 MHz LMK04616/ LMK04610

LMK00308 (1:8)

LMK00308 (1:8)

LMK00308 (1:8)

60 MHz(x8), 1 x Clk, LVDS ( AFE ADC Rx )

40 MHz(x8), 16 x Clk, LVDS ( AFE )

2.5 MHz(x8) ( AFE )

200 MHz, LVDS (FPGA Transmit) 200 MHz, LVDS (FPGA Transmit) 200 MHz (FPGA Receive)

AFE5818

7 Series

Kintex

LMK00308

CDCLVD1216

100 MHz, LVCMOS, 3.3V ( Transmit DAC)

Optional t Not applicable for system based

on pulse transmitter THS5661

CDCLVD1216 400 KHz, Power Supply

Clock Tree www.ti.com




Table 2. REF IN

VALUE80 MHz 96 MHz 100 MHz 160 MHz 200 MHz 320 MHz

2 Clock TreeExample 1: Clock tree for 128-channel ultrasound cart-based system.

Figure 1. Front End Unit and Back End Unit

Example 2: Clock tree for 128-channel cart-based ultrasound system (for front end alone) using low-power, ultra-low-jitter clock generator – LMK04610 or LMK04616

Figure 2. Front End Alone

http://www.ti.com


www.ti.com Clock Tree




Tests were conducted to demonstrate the total additive jitter and also jitter in various clock configurationsfor the clock tree by using individual evaluation modules from the different clocking components.

Figure 3. Different Clocking Components

The following graphs demonstrate the noise performance of the actual signals as measured on a signalsource analyzer.

Demonstration 1:

Reference = 80 MHz LVDS (Output signal with and without Spurs) Integrated.Integrated RMS Jitter (10 kHz – 20 MHz) – 122 fs

Figure 4. Demonstration 1, LMK61E2

http://www.ti.com


Clock Tree www.ti.com




Demonstration 2:

Reference = 80 MHz LVDSIntegrated RMS Jitter (10 kHz – 20 MHz): 208 fs

Figure 5. Demonstration 2, LMK04803

Integrated RMS Jitter (10 kHz – 20 MHz): 151 fs

Figure 6. Demonstration 2, LMK04610

Demonstration 3:

80 MHzIntegrated RMS Jitter (10 kHz – 20 MHz) – 222 fs

Figure 7. Demonstration 3, LMK0030X

http://www.ti.com


+51 fs+48 fs

+86 fs +14 fs

www.ti.com Clock Tree




A comparison of all the three demonstrates the following:• The integrated RMS jitter from the output of LMK61E2 to LMK04803 increases by +86 fs. The

integrated RMS jitter from the output of LMK04803 to LMK0030X increases by 14 fs.• The total increase in the integrated RMS Jitter from Source to destination is just 100 fs.

Figure 8. Comparison of LMK04803 to LMK0030X

Using LMK0461X series device helps reduce the total additive jitter as in Figure 9.

Figure 9. Comparison of LMK0461X Series

Using LMK0461X certain frequency combinations can also result in lower jitter as shown in Figure 10.Even if the input Jitter is high, the LMK04610 acts as a jitter cleaner and can deliver much better results –94 fs Jitter against the input jitter of 122 fs. Figure 10 has input of 40 MHz and an output of 200 MHz. Soan appropriate selection of input and output frequencies can deliver good phase noise and jitter along withappropriate Clock generator devices

http://www.ti.com


Summary and Conclusion www.ti.com




Figure 10. LMK0461X Frequency Combinations

3 Summary and Conclusion• The TI LMK series of Clock devices offer excellent additive jitter performance throughout the entire

clock chain – which are ideally suited for Ultrasound applications.• As noted above, the end to end jitter is well within the required limits of TI's high performance AFEs to

achieve optimal performance. This low end to end jitter of TI’s clock devices allows system designerssufficient headroom to take care of other factors and noise that might impact the clock performance inthe system.

• For typical Ultrasound applications it is possible to achieve superior clocking performance – byensuring very low reference clock phase noise and edge jitter.

• With the scheme and block diagram presented in the previous sections, a 320-MHz source providedthe best overall clocking system performance.

• If different reference frequencies are selected, TI provides a wide choice of other high performancedual-loop jitter attenuator/clock generator devices like the LMK046XX family – which offers additionalbenefits as:1. Lower noise floor2. Higher phase detector operating rates that reduces PLL noise further3. Better VCO noise performance4. Integrated LDOs5. Low power

http://www.ti.com


IMPORTANT NOTICE FOR TI DESIGN INFORMATION AND RESOURCES

Texas Instruments Incorporated (‘TI”) technical, application or other design advice, services or information, including, but not limited to,reference designs and materials relating to evaluation modules, (collectively, “TI Resources”) are intended to assist designers who aredeveloping applications that incorporate TI products; by downloading, accessing or using any particular TI Resource in any way, you(individually or, if you are acting on behalf of a company, your company) agree to use it solely for this purpose and subject to the terms ofthis Notice.TI’s provision of TI Resources does not expand or otherwise alter TI’s applicable published warranties or warranty disclaimers for TIproducts, and no additional obligations or liabilities arise from TI providing such TI Resources. TI reserves the right to make corrections,enhancements, improvements and other changes to its TI Resources.You understand and agree that you remain responsible for using your independent analysis, evaluation and judgment in designing yourapplications and that you have full and exclusive responsibility to assure the safety of your applications and compliance of your applications(and of all TI products used in or for your applications) with all applicable regulations, laws and other applicable requirements. Yourepresent that, with respect to your applications, you have all the necessary expertise to create and implement safeguards that (1)anticipate dangerous consequences of failures, (2) monitor failures and their consequences, and (3) lessen the likelihood of failures thatmight cause harm and take appropriate actions. You agree that prior to using or distributing any applications that include TI products, youwill thoroughly test such applications and the functionality of such TI products as used in such applications. TI has not conducted anytesting other than that specifically described in the published documentation for a particular TI Resource.You are authorized to use, copy and modify any individual TI Resource only in connection with the development of applications that includethe TI product(s) identified in such TI Resource. NO OTHER LICENSE, EXPRESS OR IMPLIED, BY ESTOPPEL OR OTHERWISE TOANY OTHER TI INTELLECTUAL PROPERTY RIGHT, AND NO LICENSE TO ANY TECHNOLOGY OR INTELLECTUAL PROPERTYRIGHT OF TI OR ANY THIRD PARTY IS GRANTED HEREIN, including but not limited to any patent right, copyright, mask work right, orother intellectual property right relating to any combination, machine, or process in which TI products or services are used. Informationregarding or referencing third-party products or services does not constitute a license to use such products or services, or a warranty orendorsement thereof. Use of TI Resources may require a license from a third party under the patents or other intellectual property of thethird party, or a license from TI under the patents or other intellectual property of TI.TI RESOURCES ARE PROVIDED “AS IS” AND WITH ALL FAULTS. TI DISCLAIMS ALL OTHER WARRANTIES ORREPRESENTATIONS, EXPRESS OR IMPLIED, REGARDING TI RESOURCES OR USE THEREOF, INCLUDING BUT NOT LIMITED TOACCURACY OR COMPLETENESS, TITLE, ANY EPIDEMIC FAILURE WARRANTY AND ANY IMPLIED WARRANTIES OFMERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF ANY THIRD PARTY INTELLECTUALPROPERTY RIGHTS.TI SHALL NOT BE LIABLE FOR AND SHALL NOT DEFEND OR INDEMNIFY YOU AGAINST ANY CLAIM, INCLUDING BUT NOTLIMITED TO ANY INFRINGEMENT CLAIM THAT RELATES TO OR IS BASED ON ANY COMBINATION OF PRODUCTS EVEN IFDESCRIBED IN TI RESOURCES OR OTHERWISE. IN NO EVENT SHALL TI BE LIABLE FOR ANY ACTUAL, DIRECT, SPECIAL,COLLATERAL, INDIRECT, PUNITIVE, INCIDENTAL, CONSEQUENTIAL OR EXEMPLARY DAMAGES IN CONNECTION WITH ORARISING OUT OF TI RESOURCES OR USE THEREOF, AND REGARDLESS OF WHETHER TI HAS BEEN ADVISED OF THEPOSSIBILITY OF SUCH DAMAGES.You agree to fully indemnify TI and its representatives against any damages, costs, losses, and/or liabilities arising out of your non-compliance with the terms and provisions of this Notice.This Notice applies to TI Resources. Additional terms apply to the use and purchase of certain types of materials, TI products and services.These include; without limitation, TI’s standard terms for semiconductor products http://www.ti.com/sc/docs/stdterms.htm), evaluationmodules, and samples (http://www.ti.com/sc/docs/sampterms.htm).

Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265Copyright © 2017, Texas Instruments Incorporated

http://www.ti.com/sc/docs/stdterms.htm

http://www.ti.com/lit/pdf/SSZZ027

http://www.ti.com/lit/pdf/SSZZ027

http://www.ti.com/sc/docs/sampterms.htm

clocking for medical ultrasound systems

Documents